This invention is concerned with improvements in or relating to molten metal inclusion sensor probes, namely sensor probes that are used in apparatus for detecting the number, size and size distribution of inclusions in molten metal, the apparatus employing what is now known as the ESZ (Electric Sensing Zone) method. The invention is also concerned with improvements in or relating to methods of making molten metal inclusion sensor probes. Such sensor probes are used in the quality control of liquid metals, such as aluminium, magnesium and steel, and are particularly valuable for this purpose in that they permit rapid on-line monitoring of flowing molten metal.
The production and refining of metals from their ores inevitably results in what, for convenience in reference, are called herein xe2x80x9cinclusionsxe2x80x9d, such as precipitated secondary phase particles, drops of slag and gas bubbles, all of which have a more or less deleterious effect upon the technical properties of the metals. An even greater quantity and variety of inclusions may be found when scrap metal is being recycled and refined, either alone or as an addition to virgin metal, owing to the presence of various products of oxidation and corrosion, dirt, oils, paint, etc, on the scrapped articles. The presence of such inclusions within the resultant rolled or cast products is generally undesirable from the point of view of properties such as fatigue life, toughness, corrosion, tearing, splitting, surface quality, pinholes, etc., particularly when larger inclusions (e.g., dimensions greater than 15 microns) are present. For example, the production of aluminium beverage can bodies is very sensitive to the presence of any inclusions within the can walls, whose thickness is of the order of 80 microns; large inclusions, which can be as large as 60 microns, can cause the metal to tear during deep drawing, or the can to perforate when its content is pressurized. Other applications in which cleanliness is critical are the production of thin sheets and lithographic plates. It is therefore essential to know whether or not the metal is sufficiently xe2x80x9ccleanxe2x80x9d for its intended purpose, and also to show whether or not the refining processes employed are producing sufficiently clean metal.
A quantitative measurement method and apparatus for such inclusions, particularly in molten aluminium, that can be operated on-line has now become well established in the aluminium industry, and is known as the LiMCA system (Trademark of Limca Research Inc.); these are described and claimed for example in U.S. Pat. Nos. 4,555,662, 4,600,880, and 4,763,065, the disclosures of which are incorporated herein by this reference. Commercial equipment is manufactured under license by Bomem, Quebec City, Quebec, Canada. The application of the method and apparatus to the detection of inclusions during the refining and recycling of other metals is under development.
The ESZ method was used prior to its application to molten metals to measure inclusions in aqueous solutions in what was known as the Coulter counter, and relies upon the fact that any inclusion usually is of different conductivity (usually much lower) than the highly electrically conductive liquid metal in which it is entrained. A measured volume of the molten metal is passed through a sensing zone consisting of an orifice of specific size (usually 300 microns diameter for aluminium) in the wall or bottom of a tube of an electrically insulating material, usually be connecting a vacuum to the tube interior, while a constant current is maintained through the sensing zone between two electrodes disposed on opposite sides of the orifice. As an inclusion particle passes through the orifice the electrical resistance of the current path through the orifice changes in proportion to the volume of the inclusion, and this change is detected as a voltage pulse between the two electrodes, or more usually between two other electrodes in the current path provided for this purpose. The amplitude of each pulse indicates the size of the respective inclusion, while the number of pulses indicates the number of inclusions in the sample volume. Besides monitoring the quality of liquid metals in terms of the number and size distribution of lower conductivity inclusions, the LiMCA system can also be used for the detection and analysis of titanium diboride (TiB2) particles that have been added to aluminium silicon casting alloys as grain refining agents. Titanium diboride is more conductive electrically than molten aluminium and voltage pulses of opposite polarity were observed.
Currently used on-line sensing probes for testing aluminium employ a sampling tube of electrically-insulating, heat-resistant material that is lowered into the metal, the tube forming a chamber into which the molten metal is sucked through a sensing zone orifice in or near to its lower end. The usual method employed at this time for forming the orifice is to drill a hole of suitable diameter through the wall of the tube, and then to heat the inlet opening using an intense micro-flame of sufficient temperature to melt the material, whereupon it flows to form a rounded edge under the action of the surface energy force that becomes operative. The current-supplying electrodes and/or the sensing electrodes may take the form of two rods disposed one inside and one outside the tube, or concentric tubes of a suitable conductive material applied to the inner and outer walls of the tube. In order for the ESZ method to operate successfully it is necessary that the electrical current path pass entirely through the electric sensing zone, and there should be no unwanted leakage between the liquid metal inside and that outside the sampling tube.
Since every particle registers a pulse when passing through the ESZ, and nonconductive particles of the same size but of different type, e.g. different density, give rise to voltage pulses of the same magnitude, it was initially impossible to discriminate between different types of inclusions within a melt. In the aluminium industry proprietary degassing units generate microbubbles and microdroplets of salt in the molten aluminium. These microbubbles and microdroplets interfere with the LiMCA probe and cause inaccuracies in its inclusion counts. In practice, microbubbles are relatively harmless compared to hard solid inclusions, and one therefore needs to distinguish one from the other from the metal quality control point of view. Better particle discrimination can be obtained by the application of DSP technology (Digital Signal Processing), which permits more information to be extracted from the signals by consideration of other parameters besides pulse height. Using the McGill DSP system each pulse can also be characterized by six other pulse parameters, namely start slope, end slope, time to maximum voltage, total signal duration, start time and end time.
Very early on the successful continuous in-line operation of the LiMCA system was found to depend on a procedure termed xe2x80x9cconditioningxe2x80x9d, which involves passing an electric current of 200-300 amperes, compared to the sensing current of about 60 amperes, through the orifice for about 300 ms before taking a new sample when it is observed that the inflow rate of the molten metal has decreased, or when instabilities are observed in the voltage baseline. The application of this high current usually is found to correct these problems, it is presumed by removing particles that have stuck to the orifice walls and are obstructing the flow of the molten aluminium and other particles through the orifice. The mechanism for this conditioning effect is a key to LiMCA""s successful implementation in melts of aluminium, and probably also in melts of other metals, but still needs be clarified. It is believed that the main mechanism has been identified and that the new sensing probe structures now provided and methods for their production renders its implementation even more effective than hitherto.
It is the principal object of the present invention to provide inclusion sensor probes for molten metals of new construction
It is another principal object of the present invention to provide new methods of making inclusion sensor probes for molten metals.
It is a further principal object to provide such probes, and methods of making such probes, with sensing orifices of improved profile which facilitates the monitoring of the particles passing in the sensing zone, and also permits prior determination of the conditioning current that is required consisting of a short pulse of high current passed through the sensing zone.
In accordance with the present invention there is provided a molten metal inclusion sensor probe of the type which is immersed in the molten metal and detects inclusions therein by the electric sensing zone method, the probe comprising a sensor probe body of electrically insulating heat resistant material having in at least a part of a wall thereof a sensing passage for the flow of molten metal from one side of the body to the other, the sensing passage providing therein an electric sensing zone and extending about a longitudinal axis;
wherein the sensing passage decreases progressively in flow cross section area from its entrance to the electric sensing zone; and
wherein the profile of the sensing passage from its entrance to the electric sensing zone is selected from, a parabola having the parabola determinant coincident with the sensing passage longitudinal axis and the focus within the sensor probe body and spaced from the sensing passage, and a segment of an ellipse having one axis parallel with the sensing passage longitudinal axis and within the body and an extension of its other axis passing through the electric sensing zone.
Also in accordance with the invention there is provided a method of making a molten metal inclusion sensor probe as specified in the immediately preceding paragraph, the method including the step of forming the sensing passage in the wall to the specified profile by an operation that will provide a passage wall smoothness of predetermined value.
Further in accordance with the invention there is provided a molten metal inclusion sensor probe of the type which is immersed in the molten metal and detects inclusions therein by the electric sensing zone method, the probe comprising a sensor probe body of electrically insulating heat resistant material having in at least a part of a wall thereof a sensing passage for the flow of molten metal from one side of the body to the other, the sensing passage providing therein an electric sensing zone and extending about a longitudinal axis;
wherein the sensing passage decreases progressively in flow cross section area from its entrance to the electric sensing zone; and
wherein the passage has been produced by a machining operation to have a wall surface smoothness of better than 1.016 micrometers (40 microinches).